Heat-curable polyurethane reactive compositions are one-component compositions, which can be cured by heating to result in a polyurethane. They comprise an isocyanate and a diol or a higher functional (meaning having more than two functional groups) polyol as an isocyanate reactive compound. Polyurethane reactive compositions allow simple manufacture of composites such as building panels and laminates, crash padding for vehicles, and reinforced structures in boats and aircrafts. They can be used as adhesives, sealing compositions, coating compositions, embedding compositions and the like. The use of polyurethane reactive compositions as adhesives is of particular interest, for example, in the automotive industry. EP-B-0062780 and EP-B-0103323 disclose the use of suspensions of solid isocyanates, which have been deactivated by superficial polyadduct formation.
EP-B-0598873 describes a polyurethane reactive composition comprising a tertiary amine or an organometallic compound as a catalyst in order to enhance the reaction rate.
A polyurethane reactive composition comprising a polyether polyol and a polyester polyol is known from EP-A-0431414.
Conventionally, the diols or higher functional polyols used in polyurethane reactive compositions are hydroxyl-terminated polyethers. Such polyethers are inexpensive and easy to handle. On the other hand, polyether polyurethanes are known to have low mechanical properties. For demanding applications where stringent mechanical requirements must be satisfied, polyurethane reactive composition comprising polyester diols or polyols yielding polyester polyurethanes are used.
Polyester polyurethanes show higher tensile strengths than polyether polyurethanes. However, polyester polyurethanes are known to show poor hydrolysis resistance. In applications where the polyurethane yielded is exposed to humidity, polyester polyurethanes are not suitable.
Epoxy resins usually show a higher hydrolysis resistance than conventional polyester polyurethanes. Further, epoxy resins often show a tensile and a lap shear strength superior to the tensile and lap shear strenght of conventional polyester polyurethanes. Due to these advantages, epoxy resins are often preferred to conventional polyurethane reactive compositions. However, cured epoxy resins are brittle. Their elongation at break is low. In applications where the performance of the material under static as well as under dynamic load is of importance, the impact resistance of cured epoxy resins is often unsatisfying. The application of epoxy resins on an e-coated substrate, for example in the automotive industry, is not suitable because of its low elongation at break and the poor impact resistance. It is therefore highly desirable to have a product, which combines a high elongation at break with a high tensile and lap shear strength and a good hydrolysis resistance.
What is needed is a polyurethane reactive composition which after curing yields a polyurethane having particularly high tensile and lap shear strength values, a high elongation at break and a good hydrolysis resistance.